1. Field of the Invention
The present invention relates to a display apparatus including a cathode ray display tube, more particularly a flat display tube comprising an envelope in which there is provided a source of electrons and a screen.
2. Description of the Prior Art
There have been many proposals for flat display tubes. Amongst these proposals is that disclosed by Texas Instruments at the Society of Information Display Seminar in 1978 and published in SID 78 Digest pages 88 and 89. The tube disclosed comprises a metal shell envelope having an optically transparent faceplate on which a cathodoluminescent screen is provided, an area cathode is provided opposite the screen and between the cathode and the screen is an apertured switching stack which in response to addressing forms and controls multiple electron beams. The area cathode comprises a plurality of stretched wire oxide coated tungsten filaments spaced approximately 3 mm apart. Field shaping electrodes are provided to obtain a uniform electron emission over the whole area of the cathode. The switching stack is operated according to an X-Y addressing system so that electrons can only pass through a selected aperture or selected apertures at any one time. Whilst this proposed tube did produce an image on the screen it is acknowledged by the engineers involved to be inferior compared to a conventional cathode ray tube due to having fewer pixels. However this proposed tube also suffered from other disadvantages, one of which is its high power consumption due to having the area cathode continuously on and only using a minute fraction of the electrons produced at any one time. Another disadvantage is that oxide coated filaments do not emit electrons homogeneously and in consequence in spite of having field shaping electrodes to equalise the distribution of electrons which are produced, it is not possible to ensure that there is an even emission of electrons across the entire area of the cathode because of the differences in emission from each of the filaments. In consequence a non-uniform image will be displayed.
It is an object of the present invention to overcome these disadvantages and to provide a display apparatus having a lower power consumption and a uniform image.